Why does my mesh have a dark outline on one of the edges?
Try following these steps:


Go in Edit Mode Tab, Vertex select 1


Select all A
Press Alt+M and select "by Distance"


With all selected still press Shift+N to recalculate the Normals


This should fix most issues.
You can check if the normals are ok by activating the "Face Orientation" here:
 

One other thing i believe you may have in your object is one or more faces inside the object, those you could find by selecting some of the outside faces, then pressing H to hide them. Allowing you to look inside the object you then can select and delete the faces inside the object with X, "Faces". Afterwards just press Alt+H to unhide all again.

Should there be still issues, please leave a comment and eventually update the question with a picture pointing it out.